The present invention relates to a driving circuit for the write head of a magnetic disk drive, and more particularly to a write driver employing capacitive boost to improve the switching performance of the circuit.
Write driver circuits are employed to provide a write current through a write head in order to record information on a magnetic disk. The write driver circuit is operable to switch the direction of the write current in a pattern that represents data to be recorded, with the switching typically performed by a circuit configuration known in the art as an H-bridge. An H-bridge is characterized by four transistor switches or variations thereof, each forming a leg of the H, with the write head coil being connected to terminals across the center span of the H. Current is directed in a first direction through the write head by operating two semiconductor switches in opposite legs of the H-bridge, and current is directed in the opposite direction through the write head by operating the other two semiconductor switches.
It is desirable for the H-bridge to switch the direction of the write current through the write head as quickly as possible, to enable a high data recording rate. The semiconductor switches of the H-bridge therefore need to be turned on as quickly as possible when they are to conduct current. One way to turn the semiconductor switches on quickly is to provide xe2x80x9covershootxe2x80x9d in the pre-drive signal that drives the transistor switches. Overshoot is defined as the amount by which the pre-drive signal exceeds its quiescent value, when a transition in write current direction is to occur. By overshooting the quiescent value when a transition is commanded, the transistor switches are turned on more quickly, decreasing the time required to switch the write current direction and increasing the rate at which data can be written. This overshoot in the pre-drive signal also causes the write current signal itself to exhibit some amount of overshoot, so that the pre-drive signal waveform and the write current waveform have similar shapes.
There are several factors which can limit the ability of the write driver circuit to provide effective overshoot of the predriver signal. One of these factors is the limit on the pulled-up base voltage of the upper switches of the H-bridge. The bases of these switches (configured as emitter-follower transistors) are conventionally pulled up to the positive voltage supply by a resistor. When the emitter-follower is pulled up maximally, the collector-base junction is unbiased. There is potential to forward-bias this junction by as much as a diode (i.e., about 0.7 volts), since the base of the emitter-follower transistor can be boosted above the supply voltage by as much as a diode before the emitter-follower transistor saturates. It would therefore be desirable to provide a circuit that allows the base voltage of the switching transistor to be boosted to a higher value than is possible with conventional designs. Such a circuit, employing a capacitive boosting technique, is the subject of the present invention.
The present invention is a write driver circuit that selectively provides write current through a write head in first and second opposite directions. First and second active devices are driven with first and second pre-drive signals. Third and fourth active devices are driven with third and fourth pre-drive signals. First and second pull-up resistances are provided respectively between the first and second active devices and a fixed voltage, and third and fourth pull-up resistances are provided respectively between the third and fourth active devices and the fixed voltage. A first capacitor is connected between the first active device and an intermediate point of the third pull-up resistance, and a second capacitor is connected between the second active device and an intermediate point of the fourth pull-up resistance. As a result of the configuration of the present invention, the high frequency response of the circuit is improved. In addition, by controlling the relative amplitudes of the high and low frequency portions of the pre-drive signals, the voltage driving the third and fourth active devices can be boosted toward (and potentially above) the positive supply voltage, improving the switching speed of those devices and increasing the overshoot (and reducing the rise time) of the write current waveform.